Question: $f(x) = \dfrac{ \sqrt{ x - 1 } }{ x^2 + 11 x + 30 }$ What is the domain of the real-valued function $f(x)$ ?
Solution: $f(x) = \dfrac{ \sqrt{ x - 1 } }{ x^2 + 11 x + 30 } = \dfrac{ \sqrt{ x - 1 } }{ ( x + 5 )( x + 6 ) }$ First, we need to consider that $f(x)$ is undefined anywhere where the radical is undefined, so the radicand (the expression under the radical) cannot be less than zero. So $x - 1 \geq 0$ , which means $x \geq 1$ Next, we also need to consider that $f(x)$ is undefined anywhere where the denominator is zero. So $x \neq -5$ and $x \neq -6$ However, these last two restrictions are irrelevant since $1 > -5$ and $1 > -6$ and so $x \geq 1$ will ensure that $x \neq -5$ and $x \neq -6$ Combining these restrictions, then, leaves us with simply $x \geq 1$ Expressing this mathematically, the domain is $\{ \, x \in \RR \mid x \geq1\, \}$.